The long-term goal of our research is to understand, in molecular terms, how cell-to-cell interactions through cell-surface receptors regulate cellular activity, with a particular emphasis on the role of protein tyrosine phosphorylation-dephosphorylation. Protein tyrosine phosphorylation by protein tyrosine kinases (PTKases) plays a prominent role in the regulation of cell proliferation and activation. The fyn, lck, and ZAP70 PTKases have been implicated in the activation and thymic development of T lymphocytes. The positive signals that are transmitted by PTKases must, however, be counterbalanced by the action of protein tyrosine phosphatases (PTPases). In T lympyocytes, CD45 (leukocyte common antigen) seems to be the most critical PTPase that regulates the T cell functions. CD45 is a family of related receptor-like transmembrane proteins found in lymphocytes and other hematopoietic cells. The cytoplasmic tail of CD45 is composed of duplicated PTPase domains. At least five distinct CD-45 isoforms are encoded by a single gene by alternative usage of three exons in the extracellular receptor domain. Various CD45 isoforms have been implicated in an array of effector and regulatory functions of lymphocytes, such as cytotoxicity and proliferation. T cell population can be subdivided on the basis of the expression of particular CD45 isoforms. In some autoimmune diseases, including systemic lupus erythematosus and progressive multiple sclerosis, distorted patterns of the CD-45 isoform expression have been observed. However, the precise mechanism by which the CD45 PTPase regulates T cell activities and/or development is not well understood. The objective of this proposal is to fill this gap in our knowledge by studying the structure-function relationship of the CD45 PTPase, using a combination of biochemical, molecular biological, cell biological, and X-ray crystallographic techniques. The specific aims of this proposal are: 1) Characterization of T cell proteins that bind to and potentially regulate the CD45 PTPase: 2) Characterization of the substrates of CD45 PTPase; and 3) Crystallographic structure determination of the CD45 PTPase domain.